Heretofore, a number of base stations have been installed as infrastructure for wireless communications (such as cellular communications). These base stations typically provide a relatively wide communication area which is called a macrocell. Because of the characteristics of radio waves, communication quality easily deteriorates locally even in a macrocell, such as an area around a boundary between the macrocell and the adjacent macrocell, indoors, and basements. To assist the base stations (hereinafter referred to as a “macrocell base station” for convenience) in such circumstances in which the communication quality easily deteriorates, small base stations have been installed to provide a relatively narrow communication area which is called a nanocell or a picocell. In addition, for example, in the High-Speed Packet Access (HSPA) system which is a 3.5 G system, femtocell base stations have been used to provide a further narrower communication area (for example, an area to cover a building), which is called a femtocell. Generally, femtocell base stations are installed in buildings which are located in macrocells of macrocell base stations.
In the cellular communication system of the 3.5 G system and later systems, the transmission speed is determined in accordance with the reception quality, based on an adaptive modulation technique. Introduction of femtocell base stations is effective for significant improvement of the transmission speed of a wireless terminal (such as a cellular phone). In addition, when high-speed cellular communication can be easily performed by introduction of femtocell base stations, significance of supporting another wireless communication system such as a wireless LAN in addition to the cellular communication, such as in a dual terminal, is reduced. In the case of supporting no other wireless communication systems, the size, cost, and power consumption in a wireless terminal can be properly reduced.
In the meantime, a femtocell is typically installed in the user's house through a contract between the user and a communication provider, and limits radio terminals, for which connection is allowed, (to, for example, wireless terminals of the contracted user) by user authentication. In consideration of such a form of use, it is estimated that the time period during which a femtocell base station is required to operate is limited. It is effective for reduction in power consumption to turn off the power supply for transmission/reception of wireless signals in a time period during which the femtocell base station is not required to operate.
As a method of managing the power supply of the femtocell base station, there is a method of always maintaining a power supply for reception in the on state, and changing the power supply for transmission from the off state to the on state when a wireless terminal of the user starts communication. Specifically, start of communication is detected by constantly monitoring a random access channel (uplink) which is used for frequency resource request. Constant monitoring of the random access channel can be realized by selecting a proper reception filter by a switch.
The above method consumes, however, electric power for reception also in most time periods during which the user does not use the femtocell base station. In particular, in the case where an interference avoiding technique called fractional frequency re-use (FFR) is used, it may be necessary to monitor a plurality of different bands for the random access channel, dependent on the regulations (the specification of the provider) of FFR. In such a case, the method incurs larger power consumption for reception, and the effect of reduction in power consumption achieved by the above method is reduced.